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Note this method is used only to compare sites with similar characteristics.

H0: Accident rate at the location under consideration in the group is equal to the average rate of the group.

H1: Accident rate at the location under consideration in the group is not equal to the average rate of the group (In another words, we are trying to find whether the site under study is “unusual” or not. We are not specifically proving it is “over-represented” or not.)

z = 1.96 for the 95% confidence level

Locations with a higher accident rate than this value would normally be selected for specific study.

Rear-end collisions are over-represented at the study site at 95% confidence level, since 14 > 10.34.

Check about LT collisions

LT collisions are not over-represented or under-represented at the study site at 95% confidence level, since 0.88<10 < 12.92.

Check about right-angle collisions

Right-angle collisions are under-represented at the study site at 95% confidence level, since 2 < 2.4.

Identifying hazardous locations and elements

Crashes happen randomly and are “rare events.” You cannot identify hazardous locations simply on the basis of the number of crashes.

A technique known as the critical crash rate factor method is one method used to identify hazardous locations. This method incorporates the traffic volume to determine if the crash rate at a particular location is significantly higher than the average for the type of facility.

We compute first the statewide CR value for similar types of roadway, which works as a “threshold.”

CR = critical crash rate, per 100 million VMT or per million entering vehicles

AVR = average crash rate for the facility type (in terms of PDO crashes; a multiplicative factor is used to convert the impact of death/injury accidents to PDO accidents)

TF = test factor, Z score (remember this needs to be a one-way analysis because we try to find a “significantly higher” or “hazardous” location) for a given confidence level,

TB = traffic base, 100 million VMT or million entering vehicles

Identifying hazardous locations and elements (cont)

Once the critical crash rate is computed, compute segment crash history in terms of PDO equivalents.

Then compute Crash Ratio as defined below:

Segment crash history

Crash Ratio =

Statewide crash history

If the resulting accident ratio is greater than 1.0, then a safety problem is likely to exist.